The magnetic resonance frequency scales with magnetic field. Accordingly, for high field magnetic resonance scanners, such as scanners with a static (B0) magnetic field of about 3 Tesla or higher, the wavelength of magnetic resonance becomes sufficiently short that the homogeneity of the radio frequency (RF) excitation, sometimes denoted as the B1 field, can become problematically inhomogeneous over a volume of interest.
A solution to this problem is to use a multi-transmit system, in which a plurality of transmit coils are operated independently. The transmit elements can be arranged as independently operable conductors of a “whole body” RF coil, or as local coils disposed on or proximate to the subject. By adjusting or shimming the relative RF power outputs of the transmit elements, the B1 field uniformity can be enhanced.
The phase relations between various channels of a multi-channel transmit system can be arbitrary, leading to arbitrary electric field cancellation or enhancement inside a subject's body. Thus, the RF power absorbed by the subject can be significantly higher in a multi-transmit than in a single-transmit system. The power absorbed by the subject is typically quantified by a parameter known as the specific absorption rate (SAR). The SAR can be computed for the subject as a whole, or for a region of the subject, and can be computed as an average SAR or as a peak SAR. If not accounted for during RF shimming, the SAR may be larger than desired, or larger than acceptable for a given magnetic resonance procedure and/or for a given subject.
A known solution is to compute the SAR as a component of the RF shimming, and to optimize both the B1 field homogeneity and the SAR simultaneously. However, computation of the SAR is computationally intensive, and typically entails modeling RF power absorption using a model of the subject including accurate information regarding electromagnetic characteristics of the organs and tissues of the subject (for a human or other biological subject). Such computationally intensive approaches are undesirable for practical applications such as diagnostic or clinical magnetic resonance imaging.
The following provides new and improved apparatuses and methods which overcome the above-referenced problems and others.